1. Field of the Invention
The present invention relates to a communication system that performs bulk data communication between information apparatuses, and more particularly to a communication system that performs data communication without interference with other communication systems between information apparatuses using electrostatic magnetic field.
More specifically, the present invention relates to a communication system that realizes bulk transmission by electric field coupling using radio-frequency (hereinafter referred to RF) signals, and more particularly to a communication system that extends a distance between electrodes of a transmitter and a receiver to be electrostatic coupled and makes an arrangement between the transmitter and the receiver and designs of communication apparatuses flexible.
2. Description of the Related Art
Recently, when data is transferred between small-sized information apparatuses, for example, when data of images, music, and the like are exchanged between personal computers, a radio interface is often used instead of a method of transferring data via data communication performed by mutually connecting the apparatuses with a general-purpose cable such as an AV (Audio Visual) cable or a USB (Universal Serial Bus) cable or via a medium such as a memory card. When the radio interface is used, it is unnecessary to reattach a connector and draw around a cable every time data transfer is performed, and thus it is very convenient for the user. Many information apparatuses implemented with various cableless communication functions are also placed on the market.
As a method of performing data transfer between small-sized apparatuses without using a cable, radio wave communication schemes for transmitting and receiving radio signals using antennas such as wireless LAN (Local Area Network) communication and Bluetooth (a registered trademark) communication represented by IEEE802.11 are developed. However, since a transmitter emits a radio wave regardless of whether there is a communication partner, there is a problem in that the transmitter is likely to be a generation source of a radio wave interfering with a communication system near the transmitter. Also, since an antenna of a receiver receives radio waves arriving from a distance, the receiver is susceptible to an interference radio wave around the receiver. The interference radio wave causes the deterioration in reception sensitivity. When there are plural communication partners, it is necessary to perform complicated setting in order to select a desired communication partner out of the plural communication partners. When plural pairs of radios perform radio communication in a narrow range, each of the pairs needs to perform frequency selection to communicate with each other in order to prevent interference among the pairs. Also, the transmitter and the receiver may be unable to communicate with each other when directions of polarization of radio waves are orthogonal.
On the other hand, as the radio communication, other than the radio wave communication performed in which the radiation-electric field is used, there are communication schemes using an induction field, an electrostatic filed, and the like. For example, in a communication system using an electrostatic field, a coupling relation is not formed when no communication partner is present nearby. That is to say, a radio wave is not radiated, and thus the communication system does not interfere with other communication systems. Also, even if a radio wave arrives from a distance, since an electrical-field coupling antenna (coupler) (hereinafter referred to as an EFC antenna) does not receive the radio wave, the communication system is prevented from being interfered by other communication systems.
Data communication techniques using an induction field or an electrostatic field have been used widely for an RFID, etc. For example, an RFID tag system has been proposed in which a set of a plurality of communication supporting bodies arranged to place RFID tags among the communication supporting bodies are formed and the RFID tags attached to a plurality of commodities are arranged to be placed among the communication supporting bodies, whereby even in a state in which the RFID tags overlap each other, it is possible to stably read and write information (see, for example, JP-A-2006-60283).
Also, a data communication apparatus has been proposed which includes an apparatus main body and wearing means for wearing this apparatus main body, includes an antenna coil and data communication means for performing data communication in a non-contact manner with an external communication apparatus via this antenna coil, has the antenna coil and the data communication means arranged in an outer case provided in an upper part of the apparatus main body, and uses an induction magnetic field (see, for example JP-2004-214879).
Also, a cellular phone has been proposed in which an antenna coil for performing data communication with an external apparatus is mounted on a memory card inserted in a portable information apparatus and an antenna coil for RFID is arranged on the outer side of a memory card slot of the portable information apparatus to secure a communication distance without spoiling portability in the RFID (see, for example, JP-A-2005-18671).
The RFID system of the past that uses an electrostatic field and an induction field has a low communication speed, because a low-frequency signal is used, and thus is unsuitable for bulk data transmission. To cope with this problem, the inventors consider that it is possible to perform bulk transmission by transmitting an RF signal using electric field coupling.
However, whereas the field intensity of a radiation electric field gently attenuates in inverse proportion to a distance, the field intensity of the induction field and the field intensity of the electrostatic field steeply attenuate in inverse proportion to the square and the cube of the distance, respectively, that is to say, the attenuation of a signal due to a communication distance is large. Also, since a propagation loss occurs according to the length of a propagation distance with respect to a wavelength, when an RF signal is propagated by electric field coupling, a problem of a propagation loss corresponding to an inter-electrode distance becomes conspicuous. It is therefore necessary to bring the electrodes (EFC antennas) of a transmitter and a receiver into close contact with each other as much as possible. Accordingly, it is difficult for the transmitter and the receiver to communicate with each other in a long distance, and thus it is not convenient for the user.
Also, in order to bring the electrodes for performing electrostatic coupling into close contact with each other, it is necessary to make a subtle adjustment on the positions of the electrodes, and the positions needs to be maintained during data communication.
Also, when an electrode for electrostatic coupling is housed in the case of a communication apparatus, it is necessary to dispose the electrode at the outside of the case as much as possible in order to reduce the distance between the electrodes at the time of data communication. Thus, the restriction is imposed on the degree of freedom of the layout in the design of the case.